1. Field of the Invention
This invention relates to integral normalized surface regions formed in situ on bulk sensitized stainless steel by laser beam scanning.
2. Description of Prior Art
Heretofore, normalized stainless steel has been made by annealing a specimen of stainless steel in the solution temperature range for a time sufficient to allow a homogenization of the chromium concentration in the stainless steel, followed by quenching of the same specimen by bringing it in contact with a cold fluid.
With a sufficiently fast quench, the homogeneous chromium concentration is retained on quenching and a normalized stainless steel specimen is obtained. However, such a bulk quenching technique is not always feasible because forming operations, welding operations or the generation of large thermal stresses may prevent such a quenching step.
Stainless steels are alloys of iron and chromium or iron, chromium, and nickel with occasionally small amounts of other elements added to enhance their corrosion resistance or mechanical properties. In regard to corrosion resistance, the chromium content appears to be the controlling variable although the effect of chromium can be enhanced by the additions of nickel and molybdenum.
Stainless steels are available in three grades: namely, martensitic, ferritic, and austenitic. The martensitic grade containing about 12 wt% chromium is distinguished by its high hardness and is used for valves, valve seats and cutlery requiring a durable cutting edge. The ferritic grade containing about 16wt% chromium is more corrosion resistant but much less hard than the martensitic grade and can be formed and drawn. The austenitic grade has a fcc structure instead of the tetragonal and bcc structure of the martensitic and ferritic grades, respectively.
The basic and most widely used grade of the austenitic type is the "18-8" type containing 18 wt% chromium, 8 wt% nickel with 0.03 to 0.20 wt% carbon. Because of its high chromium content, it has excellent corrosion resistance. In addition, because of its fcc structure, it posseses very good ductility and is used in making the seamless stainless-steel tubing used in light water reactors. 304 stainless steel is a subclass of the "18:8" austenitic grade of stainless. Its carbon content is slightly higher than the average austenitic grade.
Although 304 stainless is generally very resistant to corrosion, under certain conditions it can become "sensitized" so that it is susceptible to susceptible to catastrophic intergranular corrosion. FIG. 1 shows the equilibrium phase diagram for 304 stainless with the temperature plotted against its carbon content. It must be stressed that this is the equilibrium diagram, although the phase diagram indicates that .alpha.-ferrite can only be produced by severe cold working of the 304 stainless steel because of the extreme sluggishness of the .gamma.-to-.alpha. phase transformation. Consequently, in practice the .gamma.-austenite is retained as a metastable phase at room temperature and, thus, 304 stainless in its annealed unstrained state is austenitic.
In FIG. 1, it can be seen that the solubility of carbon in the alloy rapidly decreases with temperature between 900.degree. C. and 400.degree. C. Since most 304 stainless steels have about 0.1 wt% carbon, a super-saturated solution of carbon forms as the alloy is cooled below 900.degree. C. Given a sufficient amount of time in the "sensitization" temperature range, it is found experimentally that carbon will precipitate out along grain boundaries in the form of chromium carbide, Cr.sub.23 C.sub.6, FIG. 2. These thin two-dimensional-like carbides form on the grain boundary because the boundary is the only region where chromium atoms have sufficient mobility at these temperatures to diffuse to a carbide nucleus.
According to chromium-depletion theory of sensitization, the formation of these chromium-rich carbides along the boundary depletes the boundary and adjacent zones of chromium since at these temperatures chromium diffusion from the matrix is not rapid enough to replenish the chromium removed around the carbide. Thus, the chromium concentration at the boundary falls below that required for passivation, FIG. 2, allowing the boundary region to be corroded. A second theory also based on chromium depletion holds that the severe grain-boundary attack is a result of the galvanic cell that is formed between the bulk and the grain-boundary-zone .gamma.-austenite and that a fall of chromium concentration below that required for passivation is not necessary.
It is found that if a stainless-steel specimen is cooled rapidly through the sensitization temperature range (FIG. 1), sensitization can be avoided. However, such a bulk treatment is not always feasible because forming operations, welding operations, or the generation of large thermal stresses may prevent such a quenching operation. In such cases, other ways must be found to prevent the severe intergranular corrosion that is associated with sensitization.
An object of this invention is to provide protective coating for a body or region of sensitized austenitic grade stainless steel.
Another object of this invention is to provide a new form of normalized stainless steel that can be employed in circumstances where bulk normalized stainless steel can not be formed.
A further object of this invention is to provide a body of sensitized stainless steel with an integral surface region of normalized stainless steel formed in situ by melting and rapidly quenching the material of the surface region.
Other objects of this invention will, in part, be obvious and will, in part, appear hereafter.